You're about to meet the fit mom. Some see her toned body and everything it stands for as an inspiration to shape up. But critics believe she's condescending and out of touch. And when she asked "What's your excuse," thousands of women fired back with very real answers. So we thought we'd bring them together to square off in person. Here's ABC's Paula Farris. Reporter: From the runway to the red carpet, body fat is so in vogue. Girls willing to bear bare it all like Lena Dunham. We were saying how much we loved your bikini. I heard you laughing about it. Joan rivers. You're sending the message it's okay, stay fat, gets diabetes. Reporter: And Mindy kaling's laughing it all off. Mindy let herself go and let herself be a fat sea monster. Reporter: But fat shaming can also be serious. "Die quuyoung" singer Kesha says it's what led her to rehab for an eating disorder. ? Tonight we're going to die young ? and now the fight has spilled from the front pages to our home pages. When this mom posted "What's your excuse," it sparked reactions from tens of thousands of women. Wow, this woman is so inspiring. She should be ashamed for fat-shaming other mothers. If this is fat shaming shame away, my baby weight is coming off. Reporter: And that was excuse enough for us to meet the fit mom. Maria kang, mother of three, fitness fanatic, and accidental instigator of the latest battle of the body wars So we're going to do a little bit of legs. I've heard that I'm a bad mom. I've heard that I'm a fat shamer and a bully already. I'm just saying what's your excuse for not making health a priority? Show America how tough moms are. Go. Reporter: Maria has turned her controversial tag line into a movement. I don't think asking moms to work out is judgmental because everyone should find time in their day at least 20 minutes. Reporter: In just six months no excuse mom clubs popped up xb] But her unrelenting no excuse refrain is what drove a soft-spoken Kansas City mom to post this. You don't know till you've worked a schedule like this. Reporter: Sarah Mabin juggles a job as an overnight security guard and her day job caring for her two kids. So she routinely does 36-hour stretches with no sleep. Outraged by fit mom's post, it was Sarah and some friends who launched a "We don't need an excuse" counteroffensive. This makes me feel shamed. This makes me feel like I'm supposed to live up to a standard. Reporter: And now, after months of battling it out, they've asked fit mom to come to Kansas City and challenge her to walk in their shoes for one day. You know what? This is a tough life. Reporter: I wonder why they chose me for this assignment. I've been on maternity leave for about 2 1/2 months. I feel the pressure to get back in shape. In fact, I'm wearing a compression tank to suck it all in. First we meet up with maria in California who assures us she doesn't work out all day. She runs a chain of homes for the elderly, chases her three boys around, and even does a regular local news segment. You know, sleep is so important. Reporter: Next up, Sarah's home turf, where T moms will take it off the blog and to the kitchen table. She's pretty unapologetic. So I feel like it's going to go in one ear and out the other. Hello. Reporter: So she's not outgunned, maria and Christie, the leader of cancel city's no excuse moms club as a backup. I didn't want to come into this situation and be by myself and feel like I'm being ambushed by these moms. Can we hold hands? Reporter: With so much pent up, I don't need to do much. You don't think that in any sense maria's fat-shaming? I don't. At all. Do you? Yes. Part of the reason I think that we have so many body issues, particularly in this country, is because we're -- no. Regardless of whether you're intending shame, when people feel shamed they feel powerless. Saying that somebody needs an excuse is implying they are at fault for someone. I don't think anyone should look like me. You don't have those disclaimers on that photo. And you say you're not responsible for the thoughts that are in other people's minds. But that's very similar to a celebrity saying I'm not a role model for children. But you are. You plummeted yourself into this -- Oh, don't worry, I'm -- I always tell people that any mom's photo could have been viral but mine did and they messed with the wrong mom because I have something to say. Sure, I attracted you with my face, with my abs, with that caption. But when it comes out of my mouth about -- I love you but you're on the wrong track. That's fine. But I'm attracting other people. The difference between our two moms seems even greater. Good morning, Kansas City. It's kind of early. 20 minutes before a very long day. I didn't get the impression that she is willing to see any other side but hers. Sometimes people vilify me to the point where they think I don't know what it means to be busy. And I actually do know a lot about that. Reporter: Three adults, one child, two car seats to install. Go. Let's see how fast you can get in the car. Reporter: What better way to bring people together than a day in the family mini van? And we're off to experience a day in the life of Sarah. Which means a lot of time in here. And I just feel like oh, come on, I'm a mom. And I feel like you took my mom badge away from me. I am so sorry that I made you feel that way. Reporter: Our first stop, Diane's home schooling group. Where does the rain come from? The sky. The sky? The clouds. Reporter: Sarah contributes her time here a few times a week. I would never, ever say fitness is a bad thing or obesity is a good thing. You have to determine what works for each person. I have a 36-hour day every week. Reporter: Because of his special needs, Zaya is enrolled in activities all over town, and it's here maria thinks she sees a clear window of opportunity. Run around the block for a good half hour. It's an hour class. You can always incorporate fitness. I do not have any kind of set exercise routine. I'll walk around the field. That's awesome, actually. Reporter: For maria being a good mom means taking care of yourself. Which is why she started the no excuse mom club. Let's do this, baby. Reporter: She invited me, Sarah, and Diana to the Kansas City chapter so we could see what the movement is really all about. With the baby? Reporter: But Sarah's soon on the sidelines, unable to join in. We spent a lot of time in the car today. Do you think that maybe you could carve some time out for yourself? Well, it depends on what you're going to define as time for myself. If I took actual time to sit down and exercise, that would seem like work to me. Reporter: Diana and Sarah will say she just didn't get our point. But so many people are misinterpreting the message. Is it worth it? I think that the majority of people get the message and the small percentage that don't, I don't focus on that. You spent the last 24 hours with Diana and Sarah. Mm-hmm. What did you learn? I'm absolutely involved with Sarah and her life and especially her kids. I think we're on very different paths in our lives, and I don't see them intersecting anytime in the near future. But I have a very deep respect for what she's done. Reporter: For "Nightline" I'm Paula Faris in cancel city. .

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